Many types of polymerization processes are used in the preparation of synthetic polymers. For example, the polymerization of a monomer into a polymer can be conducted in a number of different types of reaction systems, including suspension polymerization systems, emulsion polymerization systems, nonaqueous dispersion polymerization systems, solution polymerization systems, bulk polymerization systems and vapor phase polymerization systems. Each of these systems has certain advantages and disadvantages.
In suspension polymerization systems, the initiator (catalyst) is dissolved in the monomer, the monomer is dispersed in water and a dispersing agent is incorporated to stabilize the suspension formed. All suspension polymerization processes use some type of surfactant to keep the monomer globules dispersed during the reaction in order to avoid coalescence and agglomeration of the polymer. Not only does the suspension stabilizer affect the particle size and shape, but also the clarity, transparency and film-forming properties of the resultant polymer. A variety of dispersing agents including water-insoluble, finely divided, inorganic materials and organic materials, depending upon the monomer to be polymerized, have been used as dispersing agents. Thus, for example, talc, barium, calcium and magnesium carbonates, silicates, phosphates and sulfates, as well as poly(vinylalcohol), salts of styrene-maleic anhydride copolymers, vinyl acetate-maleic anhydride copolymers, starch, gelatin, pectin, alginates, methyl cellulose, carboxymethyl cellulose, bentonite, limestone and alumina have been used as suspending agents. A major advantage of suspension polymerization is that the polymeric products are obtained in the form of small beads which are easily filtered, washed and dried. For reasons of cost and low reactivity, water is a much more desirable diluent and heat-transfer medium than most organic solvents.
However, in certain polymerization processes, the presence of moisture is highly undesirable. For example, in the preparation of cis-1,4-polyisoprene with titanium catalyst systems, the presence of significant amounts of water cannot be tolerated. Another example of a polymerization where the presence of water is highly undesirable is the synthesis of very high cis-1,4-polybutadiene with nickel catalyst systems. Thus, suspension polymerization in a water medium is not an effective process for the synthesis of cis-1,4-polyisoprene utilizing titanium catalyst systems or for the synthesis of high cis-1,4-polybutadiene using nickel catalyst systems.
An emulsion polymerization process is considered to be a three-phase reaction system consisting of large droplets of the monomer, the aqueous phase containing the dissolved initiator and the colloidal particles of monomer-swollen polymer. While the emulsion polymerization process has the economic advantage of using water as the emulsion base, not all polymerization processes can tolerate the presence of water. Such is the case with the polymerization of isoprene into cis-1,4-polyisoprene using titanium catalyst systems and with the polymerization of 1,3-butadiene monomer into very high cis-1,4-polybutadiene using nickel catalyst systems. In order to recover dry polymers which are prepared by emulsion polymerization, it is, of course, necessary to coagulate the rubber from the latex. Coagulation is generally accomplished by adding a combination of salt and acid to the latex. This results in the formation of waste water which can present environmental problems.
In solution polymerization, an organic solvent is used which is capable of dissolving the monomer, the polymer and the polymerization catalyst or initiator. Inasmuch as the polymer is soluble in the organic solvent which is used, there is a tendency for the viscosity of the solution to increase as the molecular is weight of the polymer increases. If this continues over a period of time, the solution becomes too viscous to handle in conventional polymerization reaction systems unless the solids content is limited to a low level. In commercial polymerization processes, it is desirable to obtain a polymerization mass which has a high concentration of solid polymer and, at the same time, comprises a material which is easy to handle and does not agglomerate on the walls of the reaction vessel utilized. The polymeric solution is generally steam-stripped in order to remove the solvent and unreacted monomer. The aqueous slurry of crumb rubber is usually pumped to a skimming tank, a water expeller and an extruder dryer in order to remove the water. The steam-stripping and drying operations consume a large amount of expensive energy.
The solution polymerization of isoprene monomer into cis-1,4-polyisoprene rubber with a preformed titanium catalyst system is described in U.S. Pat. No. 3,386,983. The solution polymerization of 1,3-butadiene monomer into high cis-1,4-polybutadiene rubber with a catalyst system consisting of (1) an organoaluminum compound, (2) an organonickel compound and (3) a hydrogen fluoride complex is described in U.S. Pat. No. 4,155,880.
In nonaqueous dispersion polymerizations, an organic medium is utilized which is a very poor solvent for the polymer being produced. A dispersing agent is utilized in the organic medium in order to disperse the polymer being formed throughout the medium. The dispersing agents (dispersion stabilizers) which are utilized in such nonaqueous dispersion polymerizations are generally polymeric materials which can be block copolymers, random copolymers or homopolymers. Nonaqueous dispersion polymerizations are described in detail in U.S. Pat. No. 4,098,980 and U.S. Pat. No. 4,452,960. Nonaqueous dispersion polymerization processes offer several distinct advantages over solution polymerizations and emulsion polymerizations including improved heat transfer, higher polymer concentrations in the reaction medium, increased production capacity and energy savings.
Bulk polymerization is the direct conversion of liquid monomers to polymer. Such bulk polymerizations are generally carried out by the addition of an initiator to a simple homogeneous system containing one or more monomers. The polymers produced in such bulk polymerizations can be, but are not necessarily, soluble in their own monomers which are in effect utilized as the reaction medium. For example, polyisoprene is fairly soluble in isoprene and polypentadiene is fairly soluble in 1,3-pentadiene, but high cis-1,4-polybutadiene is not very soluble in 1,3-butadiene monomer.
The synthesis of polystyrene by the addition of a free radical initiator to styrene monomer is a good example of a very common bulk polymerization. The principal advantage of a bulk polymerization process is that no solvent is utilized. Thus, the cost of solvent recovery and recycle is eliminated. One disadvantage of bulk polymerization reactions is that it is difficult to control the reaction temperature during polymerization. In fact, attempts to bulk polymerize many types of monomers have resulted in uncontrolled reaction. Due to this difficulty, bulk polymerization has not been widely utilized in the commercial preparation of synthetic rubbers.
Bulk polymerization eliminates the need for utilizing solvents which must be separated from rubber and recycled or otherwise disposed of. The cost of recovery and recycle of solvent adds greatly to the cost of the rubber being produced and can cause certain environmental problems. Recovery and separation of the rubber from the solvent also requires additional treatment and equipment, all of which further increase the cost of the rubber. The purification of solvents being recycled can also be very expensive and there is always the danger that the solvent may still retain impurities which will poison the polymerization catalyst.
The concept of preparing synthetic rubbers by bulk polymerization is not new. It has been known for many years that diene monomers can be polymerized into synthetic rubbers in the absence of a solvent. In fact, the Germans and Russians synthesized polybutadiene and polydimethylbutadiene in bulk during World War I using alkali metal catalysts in a batch process. Polybutadiene has also been prepared by the addition of catalysts to small polymerization bottles containing butadiene monomer. Due to the highly exothermic nature of such bulk polymerizations, it is not at all uncommon for the polymerization bottles being utilized in these small scale bulk polymerizations to explode. Because such bulk polymerization reactions are essentially uncontrollable, polymer uniformity is very poor, gel formation is frequently a problem and molecular weight control is very difficult. For these reasons, the bulk polymerization of isoprene monomer into cis-1,4-polyisoprene has not been widely considered to be commercially feasible.
It has been proposed to control bulk polymerizations by employing a device for cooling the reaction zone by controlled evaporation of and removal of a portion of the liquid reactant from the reaction zone. This technique is sometimes referred to as autorefrigeration. A description of bulk polymerization which employs autorefrigeration appears in U.S. Pat. No. 3,458,490. In a technique disclosed therein, a solution of polybutadiene in butadiene monomer was prepared in a solution polymerization type of reactor which was spirally agitated. However, only 35 percent of the butadiene monomer charged was converted to polymer. Steam-stripping was employed to remove unreacted monomer from the polybutadiene product formed.
Another bulk polymerization process that utilizes autorefrigeration to control foaming is described in U.S. Pat. No. 3,770,710. In a technique disclosed therein, a process was utilized which comprised initially preparing at a polymerization temperature which was not substantially in excess of about 50.degree. C., a polymer-monomer solution having a solids content of only 20 to 40 percent and continuing the polymerization of said polymer-monomer solution in a subsequent reactor at a temperature in the range of 50.degree. C. to 150.degree. C. A lithium containing catalyst is utilized in the process described therein with the Mooney viscosity of the resulting polymer increasing with polymerization time.
Canadian Patent 1,284,545 discloses a method for bulk polymerizing 1,3-butadiene into high cis-1,4-polybutadiene in a continuous process which comprises:
(1) charging said 1,3-butadiene; a catalyst system comprising (a) an organoaluminum compound, (b) a soluble nickel containing compound and (c) a fluorine containing compound; into a reaction zone; PA1 (2) allowing said 1,3-butadiene to polymerize into high cis-1,4-polybutadiene to a conversion of at least about 60 percent while utilizing conditions under which there is sufficient evaporative cooling in said reaction zone to maintain a temperature within is the range of 10.degree. C. to 130.degree. C.; and PA1 (3) continuously withdrawing said high cis-1,4-polybutadiene from said reaction zone. PA1 (1) charging into a reaction zone said isoprene and a preformed catalyst system which is made by reacting an organoaluminum compound with titanium tetrachloride in the presence of at least one ether; wherein the isoprene is maintained in the vapor phase in said reaction zone by a suitable combination of temperature and pressure; PA1 (2) allowing said isoprene to polymerize into cis-1,4-polyisoprene at a temperature within the range of about 35.degree. C. to about 70.degree. C.; and PA1 (3) withdrawing said cis-1,4-polyisoprene from said reaction zone. PA1 (1) charging into a reaction zone said isoprene and a preformed catalyst system which is made by reacting an organoaluminum compound with titanium tetrachloride; wherein the isoprene is maintained in the vapor phase in said reaction zone by a suitable combination of temperature and pressure; PA1 (2) allowing said isoprene to polymerize into cis-1,4-polyisoprene at a temperature within the range of about 0.degree. C. to about 100.degree. C.; and PA1 (3) withdrawing said cis-1,4-polyisoprene from said reaction zone. PA1 (1) charging said 1,3-butadiene and a catalyst system comprising (a) an organoaluminum compound, (b) a nickel containing compound and (c) hydrogen fluoride or a hydrogen fluoride complex into a reaction zone; wherein the 1,3-butadiene is maintained in the vapor phase in said reaction zone by a suitable combination of temperature and pressure; PA1 (2) allowing said 1,3-butadiene to polymerize into high cis-1,4-polybutadiene at a temperature within the range of 10.degree. C. to 130.degree. C.; and PA1 (3) withdrawing said high cis-1,4-polybutadiene from said reaction zone. PA1 The present invention further discloses a method for vapor phase polymerizing a conjugated diolefin monomer into a rubbery polymer in a process which comprises the steps of: PA1 (1) charging said conjugated diolefin monomer, a catalyst and a diarylamine antioxidant into a reaction zone; wherein the conjugated diolefin monomer is maintained in the vapor phase in said reaction zone by a suitable combination of temperature and pressure; PA1 (2) allowing said conjugated diolefin monomer to polymerize in said reaction zone into a rubbery polymer; and PA1 (3) withdrawing said rubbery polymer from said reaction zone.
Canadian Patent Application 2,133,526 discloses a catalyst system and technique for the gas-phase polymerization of conjugated diene monomers, such as 1,3-butadiene monomer, into polymers. However, this Canadian patent does not disclose any polymerizations of isoprene into polyisoprene. The catalyst systems disclosed by Canadian Patent Application 2,133,526 consist of (a) a rare earth metal component, (b) an organoaluminum compound, (c) a Lewis acid component and (d) an inert particulate inorganic solid. This gas phase process is reported to offer environmental advantages attributable to the fact that no solvents are used, with emissions and waste water pollution accordingly being reduced. U.S. Pat. No. 5,317,036 discloses a process and equipment for the gaseous phase polymerization of olefin monomers into polymers with unsupported soluble transition metal coordination catalysts.